1. Field of the Invention
This invention relates to a system of transferring containers between a ship and land.
2. State of the Prior Art
In transferring containers from ship to land (for example, within an automatic or semiautomatic warehouse) and vice versa, considerable improvements have been made over time in loading and unloading times of containers, enormously increasing the speed and acceleration of hoisting and translation of the cranes. Complex electronic devices have also been introduced to impose on the shifting trolley a movement law producing a damping effect on the oscillations of the container so as to reduce waiting times during unloading movements.
But a limit was reached beyond which further speed and acceleration increases made on wharf cranes of known type do not produce more appreciable benefits on the total time of unloading operations.
Attempts to increase productivity of cranes for example by hoisting in a single operation two piled or side-by-side containers. But this unloading technique imposes using cranes with capacity double that of a normal wharf crane or unloading only containers with very light loads. In addition, in case of hoisting two superimposed containers, there is a serious problem of safety since the lower container is constrained to the upper one only by the so-called ‘twistlocks’, of which it is not easy to control the effective strength and for which one cannot be sure that they will not open during the movement. Twistlocks are tools which hold together the various superimposed containers on board ships so as to constitute a single block of containers anchored in turn to the bearing structure of the ship to prevent shifting and falling of the load.
The twistlocks belong to the ship and during unloading remain hooked to the lower part of the container from which they must be removed to allow setting down the containers on land or on a land transport means and to be able to again have available the equipment of the ship for loading of new containers.
In the case of side-by-side containers, safety is greater because the double ‘spreader’ can be advantageously equipped with a control of completed hooking on both the containers. But there remain limitations due to the need that the relative position of the two containers be always the same and to the difficulty in recovering from land of two containers deposited so close by.
In the known art, cranes have been proposed with two trolleys per winch. The first winch takes a container from the ship and sets it down in a waiting position, prearranged in the crane frame, so as to free itself and be able to go and take the next container. The second winch takes the container from the waiting position and sets it on the ground after an intermediate stop for removal of the twistlocks. With this system, there is a benefit due practically only to the reduction of the rising, lowering and horizontal translation space covered by the cranes in each individual cycle.
The unloading cycle time of a container is however not satisfactory and is heavily conditioned by the time necessary for removal of the twistlocks, a time during which the crane remains stopped to await completion of an operation whose time can vary much depending on various factors such as for example, the skill of the operator, the type and state of preservation of the twistlocks and container, atmospheric conditions in which to operate, et cetera.
The general purpose of this invention is to remedy the above-mentioned shortcomings by making available a container loading and unloading system allowing high operating speeds with a substantial reduction in the time necessary to transfer a container between a ship and a warehouse while at the same time ensuring that the operators work in fully safe conditions.